Hexagonal silicon carbide platelets and preforms and methods for making and using same

ABSTRACT

Crystalline silicon carbide wherein at least 90 weight percent of the silicon carbide is formed from a plurality of hexagonal crystal lattices wherein at least 80 weight percent of the crystals formed from the lattices contain at least a portion of opposing parallel base faces separated by a distance of from 0.5 to 20 microns. The crystals may be in the form of separate particles, e.g. separate platelets, or may comprise an intergrown structure. The crystalline silicon carbide of the invention is produced by heating a porous alpha silicon carbide precursor composition comprising silicon and carbon in intimate contact to a temperature of from 2100° C. to 2500° C. in a non-reactive atmosphere. The materials are high performance materials finding use in reinforcing, high temperature thermal insulating, improvement of thermal shock resistance, and modification of electrical properties.

This is a division of co-pending application Ser. No. 899,523, filedAug. 22, 1986, now U.S. Pat. No. 4,756,895.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to silicon carbide used as a reinforcing materialfor other materials and more particularly relates to a previouslyunknown form of silicon carbide usable for this and other purposes, to amethod for the manufacture of such novel silicon carbide, to a methodfor using same, and to the resulting novel reinforced products.

2. Background of the Invention

There is presently an ongoing worldwide research effort to utilizesilicon carbide whiskers and fibers as reinforcing agents. Presentlyknown whiskers, usually single crystals having a high aspect ratio, arebased on beta phase or cubic structure silicon carbide and as a resultare not well suited for fabrication of ceramic composites which requireprocessing temperatures above 1800° C. due to the limited thermalstability of beta phase silicon carbide.

Nevertheless, attempts have been made to use such beta phase siliconcarbide as reinforcing materials.

Examples of such prior art are discussed below.

U.S. Pat. No. 4,410,635, to Brennan et al., discloses discontinuoussilicon carbide fiber reinforced ceramic composites which are formed bystarting with the ceramic matrix material in a glassy state andconverting it from a glassy state to a ceramic state after densificationof the composite.

U.S. Pat. No. 4,399,231, to Prewo et al., discloses discontinuoussilicon carbide fiber reinforced glass composites in which the siliconcarbide fibers are laid up in substantially in-plane random directionorientation through utilization of a silicon carbide paper.

.U S- Pat. Nos. 4,387,080; 4,467,647; and 4,467,042, to Hatta et al.,disclose the manufacture of flaky beta silicon carbide from an organicsilicon polymer containing a metallic or non-metallic element such asSi, B, Ti, Fe, Al, Zr, Cr, and the like. The organic silicon polymer ismolded into a thin sheet which is thereafter subjected to an anti-fusiontreatment. This sheet is thereafter heated to a high temperature in anatmosphere of non-oxidative gas such as N₂, H₂, NH₃, Ar, and CO. Thethermal treatment is carried on at a temperature not exceeding 1800° C.The resulting product is flaky beta silicon carbide. The infusible sheetof organic silicon polymer can be cut into small flaky pieces, each ofwhich has a length and breadth 10-100 times greater than the thicknessthereof, and these pieces may be converted to flakes of beta siliconcarbide. Alternatively, flaking may be effected after heat treatment ofa larger sheet. Also disclosed is the application of thin sheets orflaky materials for dispersion of thermal stress in composite materialssuch as rubber, plastics, metals, and concretes. Flaky beta siliconcarbide having a width and length in the range of 10-100 times greaterthan the thickness thereof is taught as resisting breakage in anextrusion molding machine. Such beta silicon carbide does not, however,have high temperature resistance.

U.S. Pat. No. 3,661,662, to Allen, discloses a process for making sheetsof material in which flakes of silicon carbide or boron carbide arefloated on a pool of liquid metal which is inert to the flakes andbonding the concentrated flakes together on the surface of the pool toform a sheet thereof which is withdrawn from the surface of the pool.The bonding material is an organic resin.

Alpha silicon carbide including alpha silicon carbide of a hexagonalcrystal structure is known. Such materials have not, however, beenparticularly suitable for use as reinforcing materials because in theprior art it was not possible to, or at least not practical toconsistently make silicon carbide having a structure which wassufficiently pure and flawless to act as a good reinforcing material.

Individual large, typically between 0.1 and 3 cm, and usually intergrownhexagonal crystals sometimes spontaneously appear during synthesis ofsilicon carbide by Acheson electrofurnacing. Such crystals are, however,generally too large and too few relative to total silicon carbideprepared in the furnace, to be used as reinforcing materials. Even ifsuch crystals were individually collected and crushed to smaller sized,the result would not be a good reinforcing material since such crushingoperations result in a large number of flaws in the particles of thefinished, crushed materials and also results in particles having anundesirable shape and size.

U.S. Pat. No. 3,962,406, to Knippenberg et al., discloses an inefficientmethod of manufacturing silicon carbide crystals in which a core ofsilicon dioxide is embedded in a mass of granular silicon carbide, ormaterials which form silicon carbide. Heating this mass to a temperatureat which silicon dioxide volatilizes, i.e. above about 1500° C., resultsin formation of a cavity surrounded by silicon carbide. After formationof the cavity, heating is continued at a temperature above about 2500°C., at which silicon carbide crystals of plate shape are formed on thewalls of the cavity.

Another patent disclosing the manufacture of hexagonal silicon carbidecrystals is U.S. Pat. No. RE 26,941 to Lowe. This patent describes thepreparation of large, ultrapure crystals by a slow andarduous vapordeposition process for electronic purposes for rectifiers andtransistors. The crystals are up to 0.75 inches across with thicknesses.of from 1 to 100 mils (25 to 2540 microns), see e.g. column 5, lines59-61. Such materials are generally too large for most reinforcingapplications.

American Matrix, Inc., formerly Phoenix International, of Knoxville,Tennessee, has recently announced the availability of particles of alphasilicon carbide for reinforcement of composite materials. The manner inwhich this material is made, however, has, not been published. Theseparticles under a microscope appear to be the result of crushing largeparticles. The product appears to be a mixture of various structures ofvarious shapes, e.g. needles, powder, and fragments, including somehexagonal crystal material. The particles have a large number of faultsand analysis indicates a low purity.

In summary, there are known to those skilled in the art many routes forthe preparation of alpha or beta type silicon carbide from a variety ofraw materials. However, there is no teaching or suggestion that thinsingle crystal, hexagonally shaped platelets of the more stable alphatype silicon carbide could intentionally be formed, nor is there anyteaching as to how this may be accomplished on demand, nor that suchplatelets would have any unexpected utility.

Additionally, there is no teaching or suggestion of a porous siliconcarbide matrix of small hexagonal crystal structure, i.e. where the basefaces are separated by from 0.5 to 20 microns, nor any teaching orsuggestion of a use for such a matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of the product manufactured in accordancewith Example 1.

FIG. 2 is a photomicrograph of the product manufactured in accordancewith Example 4.

FIG. 3 is a photomicrograph of the product manufactured in accordancewith Example 6.

FIG. 4, 5 and 6 are photomicrographs of the product manufactured inaccordance with Example 7.

FIG. 7 is a photomicrograph of the product manufactured in Example 8.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the present invention, there is provided crystallinesilicon carbide wherein at least 90 weight percent of the siliconcarbide is formed from a plurality of hexagonal crystal lattices whereinat least 80 weight percent of the crystals formed from the latticescontain at least a portion of opposing parallel base faces separated bya distance of from 0.5 to 20 microns.

The silicon carbide may be in the form of separate particles and atleast 70 weight percent of particles may be composed of a single crystallattice usually in the form of platelets.

Alternatively, the silicon carbide may be an integral porous structurecomprising intergrown crystals. The porous structure may comprise eitheropen or closed pore systems. The porous structure desirably has aporosity of between 5 and 80 volume percent and desirably has an averagepore diameter of between 1 and 100 microns.

The invention also includes matrices which are reinforced by theparticles of the present invention and articles formed by impregnatingthe porous structure. The matrix and impregnating materials may be glassincluding both amorphous glass and glass ceramics, ceramics, metals andpolymers. After impregnation or initial formulation, certain amorphousglasses may be converted to microcrystalline ceramics. Examples ofmetals which may be used are various steels, aluminum, and metal alloys.The reinforced matrices and impregnated products of the invention arecharacterized by good strength fracture resistance and improved thermalconductivity. In addition when matrix material is a high temperaturematerial such as sintered alpha silicon carbide, the product ischaracterized by exceptionally high temperature stability.

The silicon carbide of the present invention is produced by heating aporous silicon carbide precursor composition comprising silicon andcarbon in intimate contact to a temperature of from 2100° C. to thedecomposition temperature of silicon carbide, i.e. about 2500° C. in thepresence of a hexagonal crystal growth control additive and in an inertatmosphere, e.g. a gas selected from nitrogen, inert (noble) gases, andmixtures thereof, for a time sufficient to cause formation of saidcrystals.

The growth control additive is usually a group III A metaal and isusually selected from boron, aluminum, and mixtures thereof. The boronand aluminum may be provided in the form of boron and aluminumcompounds, e.g. Al₄ C₃, AlN, B₄ C, or BN; however, the control additiveis not the compound but is the aluminum or boron in the compound.

The growth control additive is usually present in an amount of from 0.3to 5 percent by weight of silicon in the precursor composition.

The most usual precursor compositions are selected from beta siliconcarbide powder and a mixture of particulate silica and carbon orparticulate silicon and carbon in stoichiometric amounts to form SiC.The average particle size of the precursor materials is usually betweenabout 0.005 and 5 microns: however, when a precursor material such assilica forms gaseous products prior to reaction, a much larger particlesize, e.g. 100 microns, can be used.

When separate particles are desired, a loose powder precursor is used;and when an intergrown porous structure is desired, an agglomeratedprecursor is used.

The novel products of the invention have utility in improving thestrengths of materials including stiffening in metals and toughening ofceramics. In addition the materials find utility in modifying heattransfer properties in materials, can provide improved thermal shockresistance in ceramics, can act as insulators and can improve erosionresistance in most materials. Additionally the products of the inventionhave good oxidation and chemical resistance.

DETAILED DESCRIPTION OF THE INVENTION

The silicon carbide which is the subject of this invention is alphasilicon carbide as opposed to the beta form of silicon carbide. Alphasilicon carbide is preferred in accordance with the present inventionbecause of its higher temperature resistance and better structuralintegrity.

In particular the invention comprises that form of silicon carbideformed from the "hexagonal crystal lattice," i.e. the crystal latticewhich will form a hexagonally shaped silicon carbid crystal if allowedto grow freely. While there are several such crystal lattices includingmixtures of lattice types (polytypes), which are acceptable inaccordance with the present invention, the most common are the 6H and 4Hcrystal forms. These hexagonal silicon carbide crystals defined by thehexagonal crystal lattice, when freely and completely grown, arecharacterized by two parallel hexagonally shaped "base faces" connectedat their edges by "lateral faces."

The silicon carbide crystals formed from the hexagonal crystal latticesare not always complete, i.e. the base faces are not always perfecthexagons but the crystals are usually characterized by containing atleast a portion of opposing parallel base faces which base faces areseparated by a distance of from 0.5 to 20 microns. In this disclosuresuch incomplete crystals are nevertheless of a "hexagonal crystalstructure" and the crystals are called "hexagonal crystals". The siliconcarbIde product of the invention always contains a plurality of suchcrystals, e.g. always over 1,000 and usually over 10,000.

The silicon carbide of the invention may either comprise separatehexagonal crystal particles or may be an intergrown structure comprisingintergrown hexagonal crystals.

When the silicon carbide comprises separate particles, at least 70weight percent of the particles are in the form of platelets. Theseplatelets are single crystal particles having parallel base faceswherein the longest distance across each of the base faces on theaverage is at least six and usually at least eight times the distancebetween the base faces. In a platelet the ratio of the maximum tominimum dimension of the base face is usually between 1 and 3. Each ofthe base faces of these platelets desirably contain at least twoadjacent 120° corners, which corners are characteristic of the hexagonalcrystal structure. Such corners are usually separated by a distance offrom 2.5 to 150 microns.

The particulate crystalline silicon carbide of the invention is furthercharacterized by a low flaw, i.e. fracture, rate. When viewed at amagnification of 200 power by an optical microscope, fewer than 10, andusually fewer than 2, cracks or fractures are seen per 100 basesobserved. Morphology which results from incomplete crystal growth is notconsidered a flaw, crack, or fracture.

When the silicon carbide is an integral porous structure, it comprises aplurality of intergrown crystals formed from hexagonal crystal lattices.For purposes of this invention, intergrown crystals are considered asbeing different crystals. The observable non-intercepting crystalportions continue to be characterized by having at least a portion ofopposing parallel base faces separated by a distance of from 5 to 20microns. The integral porous structure usually has a porosity of between5 and 80 and preferably between 35 and 65 volume percent and an averagepore diameter of between 1 and 100 microns. The pore structure mayeither be of the open or closed type, depending upon the desiredapplication.

The silicon carbide of the present invention is characterized by highpurity, i.e. over 95% pure and often over 99% pure. Characteristicweight percent impurities are iron lens than 0.5%, combined boron andaluminum between 0.03 and 3%, free silicon less than 0.5%, free carbonless than 0.5% and all other impurities less than 0.5% total. Most ofthe boron and aluminum impurities result from the addition of theseelements as growth control additives. Much of the residual boron andaluminum is surface contamination with internal boron and aluminumimpurities being less than their solid solution limits in siliconcarbide, e.g. less than about 3,000 parts per million for boron and lessthan about 4,000 parts per million for aluminum.

The silicon carbide products of the present invention have utility invarious reinforced structures. The products can add strength, fracturetoughness and shock resistance, while improving or maintaining thermalstability. The free platelets can be incorporated into various matricesas reinforcing aids. Such matrices include glasses including amorphousglasses and glass ceramics, crystalline ceramics, metals, and polymers.Examples of amorphous glasses include silicate glasses and glasses whichmay subsequently be converted to glass ceramics. Examples of ceramicswhich may be reinforced include silicon carbide, alumina and zirconia.Essentially all metals may be reinforced especially alloys of Iron(steel) and aluminum and other metal alloys. Such alloys may be hightemperature alloys, i.e. super alloys. Polymers which can be reinforcedby the platelets include both thermoplastic and thermoset resins, e.g.polyolefins, vinyl resins, nylons, polycarborates, and epoxys. When suchplatelets are used for reinforcement, they are usually present in anamount of about 10 to about 70 volume percent.

The intergrown porous structure may be impregnated with a material toform strong and shock resistant articles. Examples of materials withwhich the porous structure may be impregnated include amorphous glassesincluding those which may be subsequently converted to glass ceramicsand polymers and metals as previously described. Ceramics may beincorporated into the matrix by vapor impregnation.

The method for producing the silicon carbide of the present inventioncomprises heating a porous silicon carbide precursor compositioncomprising silicon and carbon in intimate contact to a temperature offrom 2100° C. to 2500° C. in the presence of a hexagonal crystal growthcontrol additive and in a non-reactive atmosphere. A "non-reactiveatmosphere" is any atmosphere comprising a vacuum or gases which,exclusive of gaseous reaction products, will not significantly adverselyaffect α silicon carbide hexagonal crystal growth. Gases which aresuitable include nitrogen and the noble gases. Oxidizing atmospheres aregenerally undesirable. The reaction is carried out for a time sufficientto cause formation of the silicon carbide crystals.

The precursor composition may be a chemical combination of silica andcarbon such as silicon carbide or polycarbosilanes in particulate orliquid form. Silicon carbide is always in particulate form includingcrystalline, whiskers, needles, or powders and is usually, but notalways, beta silicon carbide. The precursor m ay also be an intimateblend of reaction components to form silicon carbide including blends ofsilicon with carbon or silica with carbon in proper stoichiometry toyield alpha silicon carbide comprising a plurality of hexagonal crystallattices wherein at least 80 weight percent of the crystals formed fromthe lattices contain at least a portion of opposing parallel base facesseparated by a distance of from 0.5 to 20 microns. The silica, whereused, may be particulate or may be used in the form of a sol. While notwanting to be bound by any particular theory, it is believed that whensilica, silicon, or carbon are used, the silicon carbide passes throughthe beta form before forming the alpha hexagonal crystal structure.

The precursor materials provide a mixture of reactants within 10 molepercent of stoichiometry according to the equations.

    beta SiC→αSIC                                 (1)

    Si+C →beta SiC→αSiC                    (2)

    SiO.sub.2 +3C→beta SiC+2CO→αSIC        (3)

To meet such stoichiometry for equation 3, the precursor material maycontain from about 58 to about 66 weight percent silicon and 34 to 42weight percent carbon and preferably from about 60 to about 65 weightpercent silica and from about 35 to about 40 weight percent carbon.

The precursor materials must be very fine and intimately blended. Theaverage particle size of the precursor materials is usually between0.005 and 2 microns. When the precursor material is silica, much largerparticles may be used, e.g. 100 microns or higher. The reasons for thisare not completely understood, but may relate to the formation ofgaseous products containing silicon at reaction temperatures.

The precursor materials are desirably of high purity and may be obtainedfrom any suitable source.

The most common carbon source is colloidal size carbon (carbon black orlamp black). The usual particle size for carbon black is between about0.01 and 1 micron.

The silica used may be fumed silica or obtained by ashing seed hulls,especially rice hulls in air. Other seed hulls such as babassu nutshells may also be used. Silica may have a particle size as small as0.005 microns. The particle size for silica may therefore range widely,e.g. 0.005 to 100 microns or more.

A mixture of silica and carbon in about a 1:1 weight ratio may beobtained by pyrolyzing seed hulls in an inert atmosphere, e.g. ricehulls, at 800° C. Various other silica-carbon ratios can be obtained byvarying the pyrolyzing temperature between 400 and 1000° C.

In the method, the silicon and carbon in the precursor must be inintimate contact. When beta silicon carbide is used as the precursor,this contact is on the molecular level. When carbon is used as part ofthe precursor, it must be of very small particle size, e.g. a micron orsmaller and must be well mixed with the silicon source material. Thesilicon source material can usually be of a larger particle size sinceit is believed to at least partially vaporize at reaction temperature tocause contact at the molecular level.

The reaction must take place in a nonreactive atmosphere, e.g. innitrogen or in an inert gas such as argon. It is to be understood thatthe reaction may take place at subatmospheric or superatmosphericpressure. When beta silicon carbide is used as the precursor, a highvacuum may be used, eliminating the need for a gas.

Beta silicon carbide powders and whiskers may be obtained by methodsknown to those skilled in the art as for example taught in U.S. Pat.Nos. 3,340,020; 3,368,871, and 4,013,503. Desirable beta silicon carbideprecursors have average particle sizes between 0.1 and 2 microns.

The precursor must be porous, i.e. loosely packed, and where individualcrystal particles, e.g. platelets, are desired, must also be individualparticles. When an intergrown structure is desired, the precursormaterial should be agglomerated, e.g. by mixing the material in a liquidfollowed by removing the liquid and drying the product. If desired, toincrease agglomeration, a small percentage of temporary binder such as aresin may be included in the liquid. The porosity of the startingmaterial should be between 20 and 70 volume percent and the average porediameter is usually between 1 and 100 microns.

The hexagonal growth control additive is any additive which will promotethe growth of alpha silicon carbide hexagonal crystal platelets. Suchadditives are usually metals selected from group IIIA of the periodictable, especially boron, aluminum, and mixtures thereof. The growthcontrol additive when percent in the precursor is in an amount of from0.3 to 5 percent and preferably 1 to 3.5 percent by weight of silicon inthe precursor composition. The growth control additive may be a fine,e.g. 0.4 to 5 micron, powder blended into the precursor material, or maybe provided in the vapor phase during heating, e.g. by vaporization froman impregnated crucible or by vaporization from a quantity of the growthcontrol additive which is not blended with the precursor material. Whena vapor is used, the aluminum or boron is at a vapor pressure sufficientto generate material transfer from the atmosphere to the resultingsilicon carbide to form a solid solution of at least 300 parts permillion. The growth control additive may for example be placed in thebottom of a crucible and vaporized through precursor material in the topof the crucible during heating. The growth control additive may beinitially provided as a compound of boron or aluminum, e.g. Al₄ C₃, AlN,B₄ C, BN, or may be provided as metallic boron or aluminum. Regardlessof how the additive is included, it is to be understood that the growthcontrol additive is the boron or aluminum and not the compound. Unlessotherwise indicated, calculations herein are therefore based upon weightof boron or aluminum.

Any suitable non-contaminating, temperature resistant crucible may beused, e.g. crucibles made from graphite, SiC, B₄ C, or BN.

The heating temperature is between 2100° to 2500° C. A preferred rangeis between 2150° and 2400° C. The higher temperatures, e.g. above 2250°C. are usually required when nitrogen gas is used and the lowertemperatures may be used with an inert gas.

The heating time required to form the desired crystal structure,according to the methods of the invention, is usually between 3 minutesand 24 hours and is most often is at least 15 minutes. The most commonheating times are between 10 minutes and one hour.

A specific method for the preparation of the silicon carbide of theinvention is:

A. Mixing together finely divided silica, finely divided carbon, andfinely divided boron or a boron-containing compound, the amount of SiO₂to C being from about 90 to 110 percent of stoichiometric as per thereaction SiO₂ +3C=SiC+2CO, the amount of elemental boron being from 0.35to 3.5 percent based upon silicon; and

B. Heating the mixture in argon to a temperature of between 2100° and2500° C. for a time sufficient to convert the mixture into hexagonalcrystal lattice alpha silicon carbide crystals which may or may not beintergrown depending upon whether or not the initial starting materialcomprised discrete particles or was agglomerated.

The mixture is most commonly heated in a high temperature crucible, e.g.made of graphite.

A specific example for producing a rigid, porous SiC article comprisescombining 100 parts by weight of finely divided beta silicon carbidepowder, sufficient hexagonal crystal growth control additive selectedfrom elemental boron, elemental aluminum, a compound containing boron, acompound containing aluminum or mixtures thereof, to yield from 0.25 to2.5 parts by weight of elemental boron or aluminum and combining the dryresulting mix with a blend of 30 to 50 parts by weight of denaturedethanol, 5 to 8 parts by weight of polyvinyl alcohol, 1 to 2 parts byweight of oleic acid and 100 to 200 parts by weight of deionized water.The entire mix is blended in a high shear mixer and the resultinghomogenous composition is then spray dried. The dried mix is thencompacted to between 30 and 50 percent of the theoretical density ofsilicon carbide and is fired in argon at a temperature of between 2150°and 2400° C. for from 10 minutes to 1 hour. The resulting product willbe an open-pore ceramic article comprised of intergrown crystals ofhexagonal crystal lattice alpha silicon carbide crystals. The bulkdensity will be between 30 and 70 percent of the theoretical density.When this procedure is followed in the absence of the polyvinyl alcoholand oleic acid and the premix is dispersed to a low bulk density beforefiring, the result will be individual hexagonal type crystals.

Seed hulls, e.g. rice hulls or babassu nut shells, may be used forproducing single crystal platelets of hexagonal crystal lattice alphasilicon carbide. For example, specifically, rice hulls may be ashed inair at 800° C. to obtain amorphous SiO₂, and rice hulls may be pyrolyzedin an oxygen deficient atmosphere which at about 800° C. produces aresidue containing C and SiO₂ in about a 1:1 weight ratio.

The rice hulls may therefore be used as a source of silica and also ofcarbon. When a 1:1 weight ratio of silica to carbon is used, additionalsilica is required for stoichiometry. The additional silica may beprovided by any suitable source including ashed rice hulls as previouslydescribed. In such a case from about 30 to 36 parts by weight of ashedrice hulls could be comminuted with 100 parts by weight of pyrolyzedrice hulls.

The mixture would then be heated in an inert gas or nitrogen containingboron or aluminum vapor to a temperature of from about 2150° C. to about2500° C. for a time sufficient to form discrete platelet crystals ofalpha silicon carbide.

EXAMPLE 1

120 grams of beta silicon carbide powder having an average particle sizeof 0.3 microns was charged into a 21/2 inch wide by 91/2 inch long by 1inch deep graphite crucible. The powder contains less than 0.06% totaliron, less than 0.28% free silica and less-than 0.44% free carbon. Priorto charging the powders were screened through a 60 mesh cloth. The loosepowder was charged to about one inch deep. A crystal growth controladditive comprising 0.5%, based upon the weight of beta SiC charge, wasadded into the bottom of the crucible prior to addition of the siliconcarbide powder. The crystal growth control additive comprised a mixtureof 0.61g of -325 mesh metallic boron and 0.81g of -100 mesh Al₄ C₃powder.

The covered container plus charge was passed through a 6' ID graphitetube furnace in a nitrogen atmosphere with the center of the hot zonebeing 2300° C. The residence time in the hot zone was about one hour.

The resulting product was in the form of a loose powder which under anoptical microscope appears as hexagonal silicon carbide platelets havingan average maximum dimension of about 130 microns with an average aspectratio (maximum dimension divided by thickness) of about 12. The yield ofhexagonal crystals is over 95 weight percent.

FIG. 1 shows an SEM 400 power photomicrograph of typical platelets madein accordance with this example. The platelets are characterized by avery low fracture rate and high purity.

EXAMPLE 2

About 77 grams of -325 mesh aluminum oxide powder was placed in a 16ounce plastic container along with 50 aluminum oxide 0.5 inch diametercylindrical grinding media and 0.39 grams of submicron magnesium oxide.Distilled water was added to make the container three-fourths full andthe mixture was milled overnight on a roller mill.

The mixture was removed from the mill and poured into a glass dish whichwas then placed on a hot plate on low heat while stirring to prevent thealuminum oxide from getting. 20 weight percent platelets prepared as inExample 1 were gradually added as the slurry thickened. When the slurrybecame too thick to stir, the dish was removed from the hot plate andplaced in an oven at about 70° C. overnight for final drying. Theproduct was then cooled and screened through a 40 mesh screen.

The product was then hot pressed in a two and one-half inch graphitemold at 1550° C. and 2000 psig. The product was easily released from themold and appeared very uniform and dense. The final product sample was0.311 inch thick by 2.49 inches in diameter. The density was about 98%of theoretical.

EXAMPLE 3

Silicon carbide platelets prepared as in Example 1 were placed in amold. The platelets were in loose form and had a porosity of about 57volume percent. Molten aluminum alloy (89% Al, 9% Si and 2% Mg) at atemperature of about 765° C. was applied at a pressure of up to about17,500 psi during impregnation of the platelets. The resulting productwas characterized by having essentially no porosity, i.e. about 0.5% andabout 31 volume percent silicon carbide. The Youngs modulus of thecomposite material was about 20,000,000 psi as compared with theunreinforced metal at about 10,000,000 psi.

EXAMPLE 4

150 grams of beta silicon carbide powder having an average particle sizeof about 0.3 microns was mixed in a beaker with 1.5 grams of submicronboron powder.

9.5 grams of 25% polyvinyl alcohol in water, 1.5 grams of oleic acid,50cc of denatured ethanol, and 200cc water were hand stirred. Mixedsolids were then added to the mixed liquids in a blender and mixed forthree minutes.

The blended-slurry was then spray dried. 75 grams of the spray driedpowder was compacted to a volume of 63.5cc. The approximate greendensity was about 37% of theoretical. The compact was fired in argon at2150° C. for about one hour. The fired density of the resulting performwas about 45% of the theoretical density SiC with much in the hexagonalcrystalline form and with about 55% porosity. An SEM 260 powerphotomicrograph of a typical preform made substantially in accordancewith this example is shown in FIG. 2.

EXAMPLE 5

A preform prepared substantially in accordance :with Example 4 wasimpregnated with an aluminum alloy comprising 89 wt % Al, 9% Si, and 2%Mg at a temperature of about 765° C. at a pressure of up to about 17,500psi.

The resulting product was characterized by having almost no porosity andcomprised about 51 weight percent alloy and about 49 weight percentsilicon carbide.

EXAMPLE 6

About 60 grams of about 240 mesh silica, 36 grams of carbon black, and0.96 grams of submicron boron powder were stirred for 20 minutes in200cc of acetone. The mixture was dried overnight in air. The dried mixwas hand stirred and screened through a 100mesh stainless steel screen.The screened powder was then placed in a graphite crucible, covered, andfired at 2150° C. in argon.

The conversion of raw materials to SiC platelets is close to 100% oftheoretical. The average aspect ratio is between 10 and 15.

FIG. 3 shows an SEM 500 power photomicrograph on -400 mesh fraction ofplatelets made substantially in accordance with this example. The otherfractions show similar morphology and distribution.

EXAMPLE 7

125 grams of -325 mesh silicon powder was ground in a vibrational millfor 24 hours using 2200g of cylindrical 0.5 inch diameter by 0.5 highsilicon carbide media in 200cc of heptane and 8cc denatured ethanol.

The ground slurry was dried overnight. Media loss was 3.02g. Averageparticle size after grinding was 0.75 micron.

Three batches using the above ground material were made using differentgrowth control additives as set forth in Table 1.

                  TABLE 1                                                         ______________________________________                                                                      Growth Control                                  Si metal  Carbon black                                                                             Acetone  Additive                                        ______________________________________                                        1.  35 g + 1 g                                                                              15 g       150 cc 1.02 g submicron                                  media wear                  boron powder                                  2.  35 g + 1 g                                                                              15 g       150 cc 1.02 g aluminum                                   media wear                  powder -400 mesh                              3.  35 g + 1 g                                                                              15 g       150 cc 0.51 g boron                                      media wear                  powder plus                                                                   0.51 g aluminum                                                               powder                                        ______________________________________                                    

Each of the foregoing batches were mixed for 20 minutes using a magneticstirrer and dried overnight.

The samples were fired in a graphite tube furnace at from 2100° to 2150°C. for about one hour in argon.

The yield is over 90% free and intergrown hexagonal crystals. SEM 1750power photomicrographs of 1, 2, and 3 are shown in FIGS. 4, 5, and 6respectively.

These samples were analyzed by x-ray diffraction to determine crystaltype. These are shown in Table 2 to be almost entirely 4H and 6Hhexagonal type crystals.

                  TABLE 2                                                         ______________________________________                                        Sample No.  1            2       3                                            ______________________________________                                        Crystal Type                                                                  Alpha SiC 6H                                                                              Major        Major   Major                                        Alpha SiC 4H                                                                              Low          Major   Minor                                        Alpha SiC 15R                                                                             Low          Low     Low                                          ______________________________________                                    

EXAMPLE 8

Rice hulls were mixed in a blender and passed through a 20 mesh screen.

50 grams of the hulls were placed in a crucible and heated to 800° C. inflowing nitrogen to obtain amorphous SiO₂ and carbon in a ratio of about1:1.

The pyrolyzed rice hulls were ground in a vibratory mill to about-200mesh.

36 grams of the ground pyrolyzed rice hulls with 12 grams of fumedsilica were mixed for one minute in a blender.

Into a cylindrical graphite container 3 inches inside diameter by 2.5inches deep was placed 50cc a growth control additive coating suspensioncomprising 0.4 grams of submicron boron poWder and 0.55 grams -100 meshaluminum carbide powder, 0.02 grams polyvinyl acetate and 50cc ofethanol. The suspension was dried to form a film on the bottom of thecrucible.

14.5 grams of the pyrolyzed rice hull-fumed silica mix were then placedin the crucible.

The crucible was covered and heated in nitrogen in a tube furnace at amajor heat zone of 2250° to 2300° C. for about one hour.

The yield of silicon carbide platelets was about 75 percent oftheoretical. Platelets prepared substantially in accordance with thisexample are shown in the 400X optical photomicrograph of FIG. 7.

The average size of the platelets is less than 150 microns and theaspect ratio (maximum dimension/thickness) is between about 15 and 20.

EXAMPLE 9

3.55 grams of silicon carbide platelets having a size fraction of-60/+200 mesh were wet mixed with 20 g of submicron alpha SiC powder,0.13 g of B₄ C and 1% phenolic resin as a carbon source and subsequentlydried.

A 1-inch diameter disc was prepared by compacting 10 g of the mixture at20,000 psi to a green density of 55% of theoretical. Pressurelesssintering was conducted at 2150° C. in an argon atmosphere with a 21/2hour hold at temperature. The resulting sintered density was 82.7% oftheoretical. The fracture surface of this sample was studied by SEMshowing that the platelets fully survived in the matrix.

What is claimed is:
 1. A method of producing hexagonally shapedplatelets of alpha silicon carbide comprising:A. Mixing togetherpowdered silica, finely divided carbon of from 0.005 and 2 micronsaverage particle size and finely divided boron or a boron-containingcompound of from 0.4 to 5 microns average particle size, the amount ofSIO₂ to C being from about 90 to 110 percent of stoichiometric as perreaction SIO₂ +3C=SiC+2CO, the amount of elemental boron being from 0.35to 3.5 percent based upon silicon; and B. heating the mixture in argonto a temperature of between 2100° and 2500° C. for a time sufficient toconvert the mixture into hexagonally shaped platelets of siliconcarbide.
 2. The method of producing a crystalline silicon carbideproduct wherein at least 90 weight percent of the product is in the formof a plurality of hexagonal crystal lattices and wherein at least 80weight percent of the crystals formed from the lattices contain at leasta portion of opposing parallel base faces separated by a distance offrom 0.5 to 20 microns, said method comprising heating a porous siliconcarbide precursor composition comprising silicon and carbon in intimatecontact to a temperature of from 2100° C. to 2500° C. in the presence ofa hexagonal crystal growth control additive and in a non-reactiveatmosphere, for a time sufficient to cause formation of said crystals,wherein the growth control additive is selected from boron, aluminum andmixtures thereof and wherein the growth control additive is present inan amount of from about 0.3 to about 5 percent by weight of silicon inthe precursor composition and wherein the precursor composition is anintimate mixture of from about 58 to about 66 weight percent silica withfrom about 34 to 42 weight percent carbon and wherein the silica isobtained by ashing seed hulls in air.
 3. The method of claim 2 whereinthe seed hulls are rice hulls.